paper: 4 CIM versus 6 CIM theoretical calculations

[AdamHeard]

Originally Posted by GeeTwo

I agree that there would not be a problem while driving the robot, but there might be an issue when on blocks. I figure (using Vex's numbers and a bit of linear extrapolation) that putting a CIM and mini-CIM nose to nose at 12V would reach a free speed of 5507 rpm, at which point the mini-CIM would be drawing 7.9A, 46W of mechanical power would be transferred, and the CIM would be generating 1.6A. The CIM would switch from consuming to generating current at about 5442 rpm, so probably even a gearbox on blocks would provide enough drag. Of course, both the CIM and mini CIM free speeds have a +/- 10% variation, so if you put a fast mini and a slow CIM together, you might have some issues when running the motors with the robot on blocks.

This wouldn't cause any issues, you'd just be pumping some current back to the DC bus and everything would be fine.

[InFlight]

Electrical Power and Mechanical Power are not the same.

The Electrical Power into the a DC Motor is Volts x Amps (Watts)

The Mechanical Power Out is:

Power Out = Torque*(0.112985) * Speed * (2*pi/60) in Watts

Note: 1 lb-in = 0.112985 Nm, and radian/sec 2*pi/60 = rpm

The Motor Efficiency is

η=(Power Out) /( Power in)

At near free speed, the torque and Mechanical Power approach zero.

[InFlight]

For any of the DC motors used in First Robotics, we can also use the following equations to determine their performance:

Current (Amps) = Torque Load * ((Stall Current- Free Current)/Stall Torque) + Free Current

Torque Load = (Current-Free Current) * Stall Torque/(Stall Current-Free Current)

Speed = Free Speed - (Free Speed / Stall Torque) * Torque Load

[GeeTwo]

Originally Posted by AdamHeard

This wouldn't cause any issues, you'd just be pumping some current back to the DC bus and everything would be fine.

If you didn't have motor controllers, or (illegally) put both motors on the same controller, this would certainly be the case. Are motor controllers designed to handle the reverse bias? - Oh, right. If you decide to change from 100% speed to 50% speed in a short period of time, you'll create the same situation, so they have to be designed that way. Never mind.

[MrRiedemanJACC]

Originally Posted by Richard Wallace

This is not correct. Check the data, which I can vouch for. It was measured correctly by people who know their motor physics, and their FRC design.

The Mini CIM has about 2/3 the active material (armature core length, permanent magnets) compared to the CIM, and it has the same commutator.

This is why the Mini CIM performs well during prolonged heavy loading -- it does not heat up as fast internally as a CIM under the same load proportional to its size. Look at the test results provided by VexPro; after 60 seconds at peak load, the Mini CIM is still providing 200 Watts shaft output (87% of what it developed starting out with room-temperature innards), while the CIM is down to 230 Watts shaft output, only 70% of what it developed cold. Pound for pound in the heat of combat, the Mini CIM outperforms its big brother.

So I'm a dumb Mechanical Engineer and last year we had a 3 Cim per side shifting drivetrain with tracks. Obviously we could have swapped out a Cim for a Minicim to help out with shooter power and other items. (we didn't and maybe should have). But let me ask this question, How does a Cim plus 2 Minicim drivetrain compare to a 2 cim (per side) in tank drive? Assuming the rules are similar this year with open availability of other motors, would it make sense to keep the 4 cim motors for whatever gamepiece needs there are? (oh and I know I'm asking for everyone to predict the future!)

[Knufire]

For those debating the merits of the 1:1 gearing, you might want to take a look at this thread.

https://www.chiefdelphi.com/forums/s...d.php?t=123424

To address the benefits of 4 CIM vs 6 CIM vs 4CIM+2 Mini-CIM, 234 has a paper published from a few years back with experimental data.

https://www.chiefdelphi.com/media/papers/3071

[GeeTwo]

Originally Posted by MrRiedemanJACC

So I'm a dumb Mechanical Engineer and last year we had a 3 Cim per side shifting drivetrain with tracks. Obviously we could have swapped out a Cim for a Minicim to help out with shooter power and other items. (we didn't and maybe should have). But let me ask this question, How does a Cim plus 2 Minicim drivetrain compare to a 2 cim (per side) in tank drive? Assuming the rules are similar this year with open availability of other motors, would it make sense to keep the 4 cim motors for whatever gamepiece needs there are? (oh and I know I'm asking for everyone to predict the future!)

While 1C+2m would certainly have better performance than a 2 CIM drive train, that performance gain is offset by a greater weight and volume, both of motors and of the gearbox - 3 motor gearboxes usually have a higher center of gravity or more gears than a 2 motor gearbox (or both). There's also the matter of another motor controller. There are certainly cases where this would be better, but I expect they're less common than the configurations already discussed.

[cbale2000]

Originally Posted by GeeTwo

While 1C+2m would certainly have better performance than a 2 CIM drive train, that performance gain is offset by a greater weight and volume, both of motors and of the gearbox - 3 motor gearboxes usually have a higher center of gravity or more gears than a 2 motor gearbox (or both). There's also the matter of another motor controller. There are certainly cases where this would be better, but I expect they're less common than the configurations already discussed.

I'll second this, it certainly wouldn't be the first choice for motor configurations IMO. In general there's rarely a need to use more than 2 CIMs outside of the drive system anyways, so the number of instances you would actually need to swap out to a 1CIM+2MiniCIM should be minimal (short of maybe saving a little weight if you already have a 2CIM+1MiniCIM drive and desperately needed to loose around 1lb)